Trunked mobile communication systems are known. Such systems typically allocate communication resources (such as a frequency pair or TDM time slot) upon perception of a need for communication services. Such communication resources typically allocated on a 30 KHz bandwidth channel in allocated areas of an assigned frequency spectrum.
In some systems perception of a need for communication services occurs when a mobile communication unit transmits a resource request to a resource controller. In other systems a communication unit simply seizes control of a communication resource and begins transmitting a signal.
In either case the geographical range of a mobile communication unit is limited. The limited range often requires that communication transactions between units, or between a mobile unit and a PSTN subscriber, be supported through central site communication support facilities. Central site communication support facilities (base sites) provide communication support functions within a service coverage area proximal to a base site antenna.
Upon entering the service coverage area of a base site a communication unit may request service through a resource controller that may be located at the base site. The communication unit in desiring service may first search a designated frequency spectrum for a control resource (an uplink) over which to transmit a service request. The communication unit then monitors the resource (a downlink) for a response. Where the control resource consists of a frequency pair the uplink typically consists on the first of the pair and the downlink is the second frequency of the frequency pair.
Upon activation a communication unit searches for a control resource. Upon finding a control resource, on the other hand, the communication unit may wish to determine whether the resource represents the nearest resource controller thereby insuring adequate reception. The communication unit makes this determination by measuring the signal strength of the transmitted signal on the control resource. The communication unit may make such a measurement on a number of frequencies (from differing base sites) before selecting the resource offering the strongest signal, thereby insuring access to the nearest base site.
After transmitting a resource request on an uplink through the nearest resource controller the communication may wait for a response. The resource controller may respond with an information signal identifying a different communication resource over which the communication unit may conduct a communication transaction. Upon receiving such information the communication unit tunes to the assigned frequency and begins communicating.
Subsequent to initiating a communication transaction, and as the transaction continues, a communication unit may move outside the service coverage area of the base site. The communication unit detects such condition by monitoring the signal received from the base site. As the signal level received from the base site exceeds a threshold, the communication unit scans communication resources for a control resource used by another nearby base site. Upon detecting another base site, the communication unit requests a hand-off to the adjacent base site. If a is available within the other system a hand-off may occur, if not, the call may be dropped.
Transimssions between communication units and base sites are, in many systems, under a frequency division multiple access (FDMA) format. Other systems use time division multiplexing (TDM) in conjunction with the FDMA format.
Spread spectrum communication systems are also known. Spread spectrum systems are communication systems with a high inherent degree of resistance to interference from other transmitters. Spread spectrum systems achieve their resistance to interference by transmitting an information signal distributed over a relatively wide frequency bandwidth. Use of the wide bandwidth avoids a total blocking of signals from relatively narrowband interferers present on individual frequencies or limited bands of frequencies.
Two examples of spread spectrum systems are direct sequence spread spectrum systems (DSSS) and frequency hopping. DSSS systems achieve the wide band distribution of a message signal by encoding an information signal with an output from a pseudo random number generator. Frequency hoppers, on the other hand, achieve a distribution of signals through an indexing format.
The pseudo random number generator used by a DSSS system is a digital circuit used to generate a repeating sequence of digital numbers under the control of a code key. The use of a code key allows a receiver having an substantially identical pseudo random number generator as the transmitter, and the appropriate code key, to decode a transmitted signal. Other receivers without the proper codes are unable to decode the signal. Without the proper code, in fact, a receiver may not even by able to detect the presence of the spread spectrum signal.
The inability of a spread spectrum receiver to detect a spread spectrum signal transmitted under a different code key and the immunity of spread spectrum systems to interference are together the greatest asset of the spread spectrum technology. Because of the great immunity of spread spectrum receivers to competing signals relatively large numbers of transmitters and receivers using different code keys can share the same frequency spectrum with relatively little mutual interference but with a significant increase in spectral efficiency resulting from the greater number of users.
Because of the inherent resistance of spread spectrum communication systems to interference a need exists for a way of applying spread spectrum to trunked communication systems. Such a system, on the other hand, must provide a methodology of detecting spread spectrum signals transmitted under a number of different code keys.